The present invention relates to a smooth exhaust valve connected between a vacuum chamber and a vacuum pump and used for gradually reducing pressure in the vacuum chamber to vacuum pressure.
In general, if gas at atmospheric pressure or high pressure in a vacuum chamber is exhausted rapidly in reducing internal pressure of the vacuum chamber to vacuum pressure by a vacuum pump in a manufacturing process and the like of a semiconductor, a large amount of gas flows temporarily. As a result, gas turbulence occurs in the vacuum chamber, a cloud of particles deposited on a vacuum chamber inner wall or the like is raised, and the particles may be deposited on a workpiece.
Therefore, a vacuum valve having a main valve body having a large opening area and an auxiliary valve having a small opening area is disposed in a flow path connecting the vacuum chamber and the vacuum pump. After carrying out initial exhausting by applying air pressure to a piston of the auxiliary valve body to open the auxiliary valve body, air pressure is applied to a piston of the main valve body to open the main valve body to thereby change a gas inhalation amount in stages. In this case, opening accuracy of the auxiliary valve in the initial exhausting is extremely important in order to avoid gas turbulence in the vacuum chamber and precise control of an opening degree of the auxiliary valve is required.
In the above vacuum valve, on the other hand, the main valve body and the auxiliary valve body are respectively and constantly repulsed in valve-closing directions by return springs and the respective valve bodies open when air pressure operating forces applied to the pistons exceed the repulsing forces of the return springs.
However, because a repulsing force characteristic of the spring generally varies from product to product, it is extremely difficult to make a spring load applied to the valve body in every vacuum valve uniform when the spring is used as the above-described return spring for the main valve body or the auxiliary valve body. Especially in a case of the auxiliary valve of which high opening/closing accuracy is required, if the characteristic of the return spring varies, a deviation is generated in a control curve of the vacuum valve, which affects an input/output characteristic. Therefore, the return spring has to be replaced by one having spring accuracy conforming to specifications. However, because the return spring for the auxiliary valve is normally disposed in a narrow space inside the valve, the spring cannot be replaced without disassembling to fine parts in many cases and a replacing operation is extremely complicated. Moreover, because it is necessary to readjust a relationship between air pressure and a returning force of the auxiliary valve after the replacement, there are problems of enormous amounts of effort and time required before the vacuum valve is brought into operation and high cost.
It is an object of the present invention to provide a smooth exhaust valve in which a repulsing force of a return spring for an auxiliary valve body can be easily adjusted by an operation from outside without replacing the spring even if a repulsing force characteristic of the spring is slightly different from specifications to thereby accurately and stably control opening/closing of the auxiliary valve body.
To achieve the above object, according to the present invention, there is provided a smooth exhaust valve comprising: a first port to be connected to a vacuum chamber; a second port to be connected to a vacuum pump; a main flow path having a large cross-sectional area and an auxiliary flow path having a smaller cross-sectional area than the main flow path for connecting the first port and the second port in parallel; a main valve body for opening and closing the main flow path; an auxiliary valve body for opening and closing the auxiliary flow path; a first piston connected to the main valve body through a first shaft; a second piston connected to the auxiliary valve body through a second shaft; a main pressure operating chamber for applying air pressure in a valve-opening direction to the first piston; an auxiliary pressure operating chamber for applying air pressure in a valve-opening direction to the second piston; first spring means for repulsing the main valve body in a valve-closing direction; second spring means for repulsing the auxiliary valve body in a valve-closing direction; and an adjusting rod mounted to a valve casing so as to be operated from outside to move forward and rearward and for changing a compressed amount of the second spring means by operation of forward and rearward movements of the adjusting rod to adjust a repulsing force of the second spring means.
According to the invention having the above structure, because the repulsing force of the second spring means can be adjusted by changing the compressed amount of the second spring means by operating the adjusting rod by forward and rearward movements even if the repulsing force characteristic of the second spring means is slightly different from the specifications, it is unnecessary to replace the second spring means.
According to a concrete embodiment of the invention, the auxiliary flow path and the auxiliary valve body are incorporated in the main valve body, the second piston and the auxiliary pressure operating chamber are incorporated in the first piston, the second shaft is incorporated in the first shaft, the second spring means is provided between the second piston and a movable spring seat in the first piston, and the adjusting rod is screwed for moving forward and rearward down into the first piston with a tip end portion of the adjusting rod in contact with the movable spring seat and a base end portion of the adjusting rod projecting outside from the valve casing.
According to an embodiment of the invention, third spring means is provided in the first shaft as spring means for repulsing the auxiliary valve body in the valve-closing direction in addition to the second spring means.
According to another embodiment of the invention, the first piston has a connecting hole for connecting the main pressure operating chamber and the auxiliary pressure operating chamber and the valve casing has one operating port for supplying pressure air to the main pressure operating chamber.
According to yet another embodiment of the invention, the valve casing has first and second operating ports for individually supplying pressure air to the main pressure operating chamber and the auxiliary pressure operating chamber, the first operating port communicates with the main pressure operating chamber through a through hole provided to the valve casing, and the second operating port communicates with the auxiliary pressure operating chamber through a through hole provided in the valve casing and the first shaft.
In the invention, the main valve body is a poppet valve and the auxiliary valve body is a needle valve.